Electrochromism refers to a reversible optical spectrum change upon an electron transfer reaction induced via an application of voltage. Recently, electrochromic (EC) materials play an increasingly vital role in both academic and practical areas since it achieves a reversible and highly stable variation of transmissive/absorptive spectra and tunable colors between doped and undoped states, and hence possesses a great potential to be used in smart windows mirror, sunglasses, digital signage and displays, as well as electronic paper.
Among various EC material candidates, inorganic materials such as (transition) metal oxides and their complexes are the most widely exploited color-changing materials due to broad polaron absorption and high photochemistry, nevertheless, always limited by a slow respond time and low coloration efficiency. A polaron is a quasiparticle used to understand interactions between electrons and atoms in a solid material. A photochemistry refers to a chemical reaction caused by absorption of light. Alternatively, conjugated polymers have been recognized as desired materials in all sorts of electrochromic devices (ECDs) for their tunable colors, high optical contrast, easy processability, and long-term stability. Currently, many investigations have been devoted into the various saturated colored-to-transmissive polymeric ECDs from their neutral states to doped states. Especially, black-to-transmissive electrochromic polymers (ECPs) is gradually becoming a hot spot as it is signified to be a promising material for privacy glass and smart windows. However, there still remains a challenge due to complexity and incompleteness of absorption over the whole visible spectrum in the neutral state, followed by bleaching out in a fully oxidized state.
Up to now, significant efforts have been devoted to design and obtain a desired black color and a full spectrum, and various materials and strategies are emerging. As mentioned, although metal oxides and their complex, such as porous NiO, Co-based polymer, CuO2, IrO2 have been extensively investigated and can be served as the black-to-transmissive electrochromes, they show inferior EC performances (low coloration efficiency and slow color change) compared with organic polymer materials.
Organic black-to-transmissive EC materials include (1) individual copolymers absorbing the entire visible spectrum, and (2) complementary compositions based on “color-mixing” theory to achieve a complete spectrum and a black-to-transmissive EC display. When two or more different monomers unite together to polymerize, their result is called a copolymer and its process is called copolymerization. Various sorts of black-to-transmissive ECDs are reported with the second method by blending multicolored compounds or constructing complementary multilayers. Consequently cathodically coloring layer EC materials can potentially complement an anodically coloring electrode to form a panchromatic absorption spectrum within the visible spectrum. However, the resulting colors obtained by this method show a relatively low optical contrast, complex process, or pseudo-color phenomenon. Also, performance and manipulation limitations seriously limit the applications in a narrow range.
Initiated by others works on the design and synthesis of conjugated ECPs with neutral state colors of blue, green or black, band gaps of these materials must be lower than 1.75 eV. To construct a low-band gap system, a “donor-acceptor” (D-A) strategy via alternating electron-donating and electron-accepting (i.e., electron-rich and electron-deficienr) units, is verified as an effective solution and has been widely employed to synthesize individual copolymers. Nevertheless, the band gap is not the only factor in determination of a color state. A color state is a comprehensive result of wavelengths and intensities with different optical transitions. More importantly, there are multiple absorption spectra with various breadths existing in the D-A copolymers structure. The first neutral-state black-to-transmissive polymeric electrochrome was proposed and developed by Reynolds group (P. M. Beaujuge, S. Ellinger, J. R. Reynolds, the donor-acceptor approach allows a black-to-transmissive switching polymeric electrochrome, Nat. Mater. 2008, 7, 795-799). Then they made further improvements and investigated several kinds of copolymers in lower fabrication and processing costs, as well as good performance, exhibiting fast switching responds, high optical contrast. Nevertheless, additional improvements are still required for more uniform and broad absorbance across the entire visible spectrum to meet needs of the applications.